Meteorological and glaciological data obtained over an intensive 2 year measurement period (2000–2002) are used to run a physically based climatic mass balance model to characterize a seasonal variability in mass and energy exchanges at Summit, Greenland. The model resolves the full surface energy balance and the subsurface temperature profile by inclusion of energy release from penetrating shortwave radiation. A Monte Carlo approach using 1000 different parameter combinations is adopted to assess model uncertainty, with output compared to measured surface and subsurface temperatures, changes in surface height, and eddy correlation data. The heat exchanges associated with the change in phase of water are very small in all seasons, with the average turbulent latent heat flux equal to 0.4 (±0.2) W m−2. This suggests that the mean annual water vapor gradient is toward the surface, resulting in a mass gain of 4.1 mm WE yr−1. The mass gain represents only a small fraction of the total accumulation (<2%), in part because of the change in sign of the water vapor flux from winter (deposition) to summer (sublimation), but if assumed to be typical of the entire dry snow zone (40% of the total ice sheet area) is equivalent to approximately 5.5 Gt yr−1. A simple experiment based on 2012 atmospheric conditions suggests that mass turnover from water vapor exchanges will likely be enhanced in a warming climate, with sublimation increasing more than deposition. Should the sign of the mean turbulent latent heat flux change due to warming, the present mass gain in the dry snow zone could easily become a mass loss of equal proportion, which would further enhance the negative mass balance of the Greenland ice sheet.